Excess Shear Stress Output
 

Hello Flow3D users,
How exactly Flow3D calculates the "Excess Shear Stress" output?
Thank you for your attention.

If the flow is turbulent (a turbulence model is active) then the cells that include some solid surface are assumed to have a logarithmic velocity distribution. The distribution is derived from the work of von Karman, Nikuradse, and a lot of later researchers. There's a smooth function that adjusts the shape of the profile for whether the near-solid flow is hydraulically smooth, transitionally rough, or fully rough. The "log law of the wall" can be represented as a plot with y+ on the x-axis and u+ on the y-axis. Both y+ and u+ have shear stress in them, so the solution is iterative and coupled with the velocity field, which is in turn coupled with the pressure field. The FLOW-3D Users Site has several technical notes that fill in the rest of the details.

Thank you for your answer.
I have another doubt. Is it possible to obtain shear stress value at specific points determined by the user? So far I can only obsverve excess shear stress graphical results.

Yes. Before you run, you can add History Probes from the Meshing & Geometry tab. They are x y z points that record data. Put them close to the bed (in the same cell as the bed) and they will report shear.

Or, after the run, use Analyze > Text Output and get a box-shaped region of cell-centered shear stresses.

Or, after the run, make a text Neutral file (see the User Manual for the format) with x y z coordinates for data and use Analyze > Neutral File.

You can also get average shear stress on a solid by turning on Component Properties > Output > Pressure & Shear Force output and Wetted Area output. Then run the case, find the output in Analyze > Probe > General History Data, and take (component shear stress magnitude)/(component wetted area) to find the average shear stress.

Hi,
So, How does Flow-3D compute bed shear stress in separated flows where Log-law of the wall in not valid ( for k-e model)?

Thanks in advance
Mahdi